H₂O activity in albite melts at deep crustal P-T conditions derived from melting experiments in the systems NaAlSi₃O₈-H₂O-CO₂ and NaAlSi₃O₈-H₂O-NaCl

Abstract

The system NaAlSi₃O₈ (albite, Ab)-H₂O offers a simple and tractable model to study the thermodynamics of the volatile constituent H₂O in felsic magmas. Although it has been studied in this context for nearly 100 years, developing a comprehensive model that adequately describes the activity of H₂O (${a_{{H_2}O}}$) in hydrous albite liquids and vapors has proven challenging. There are several problems. First, ${a_{{H_2}O}}$ in hydrous liquids relies on melting experiments in the presence of mixed fluids with reduced H₂O activity (H₂O-CO₂ and H₂O-NaCl), but models of ${a_{{H_2}O}}$ in these coexisting fluids have lacked sufficient accuracy. Second, the role of the solubility of albite in H₂O has been assumed to be negligible; however, it is important to take solubility into account at pressure (P) above 0.5 GPa because it becomes sufficiently high that H₂O activity at the wet solidus is significantly less than 1. Third, the dry melting temperatures and wet solidus temperatures are inconsistent between the datasets. We address these issues by combining previous experimental work on T–${X_{{H_2}O}}$ liquidus relations at 0.5–1.5 GPa with accurate activity formulations for H₂O in mixed fluids (Aranovich and Newton, 1996, 1999). This yields isobaric T–${a_{{H_2}O}}$ sections at 0.5, 0.7, 1.0 and 1.5 GPa. Data at each isobar were fit to cubic equations, which were used to derive the following equation for liquidus T as a function of ${a_{{H_2}O}}$ and P:

$$T\left( {{a_{{H_2}O}},P} \right) = {m_0} + {m_1}{a_{{H_2}O}} + {m_2}a_{{H_2}O}^2 + {{m_3}a_{{H_2}O}^3}^\circ C$$

where T is °C, m ₀ = 1119.6 + 112.3P, m ₁ =–856.5–578.9P, m ₂ = 1004.1 + 952.9P, and m ₃ =–477.1–618.0P. The equation is valid at 0.5 < P < 1.5 GPa and T _solidus < T < T _{dry melting}. The nonzero solubility of albite in pure H₂O is incorporated into the model to give the correct liquidus H₂O activity when truncating the model equation in the limiting case where T → T _solidus at a given pressure. This model equation reproduces both the liquidus-H₂O contents and activities from the solubility measurements of Makhluf et al. (2016) in the binary system Ab-H₂O at 1.0 GPa. The model equation also accurately reproduces the liquidus H₂O activities from Eggler and Kadik (1979) and Bohlen et al. (1982) when the Aranovich and Newton (1999) activity formulation for CO₂–H₂O mixed fluids is applied to their datasets.